Thermal spraying, also known as flame spraying, involves the heat softening or melting of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A conventional thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically formed of small particles, e.g., between 100 mesh U.S. Standard screen size (149 microns) and about 2 microns.
A thermal spray gun normally utilizes a combustion or plasma flame to produce the heat for melting of the powder particles. In a powder-type combustion thermal spray gun, the carrier gas, which entrains and transports the powder, can be one of the combustion gases or an inert gas such as nitrogen, or it can be simply compressed air. In a plasma spray gun, the primary plasma gas is generally nitrogen or argon. Hydrogen or helium is usually added to the primary gas, and the carrier gas is generally the same as the primary plasma gas.
One form of powder for thermal spraying is a composite or aggregated powder in which very fine particles are agglomerated into powder particles of suitable size. Such powder produced by spray drying is disclosed in U.S. Pat. No. 3,617,358 (Dittrich) which also teaches various useful polymeric (organic) binders for the agglomerating. Agglomerated powder also may be made by blending a slurry of the fine powder constituents with a binder, and warming the mixture while continuing with the blending until a dried powder of the agglomerates is obtained. Generally the binder for the blending method may be the same as disclosed for spray drying.
U.S. Pat. No. 5,049,450 (Dorfman et al) teaches a homogeneous thermal spray powder produced by blending with a binder in a slurry, the powder being formed of subparticles of boron nitride and silicon-aluminum alloy. This patent is directed particularly to a powder for producing thermal spray coatings that are abradable such as for clearance control applications in gas turbine engines. The boron nitride is not meltable and so is carried into a coating by the meltable metal constituent and the binder in the thermal spray process. Excellent, abradable coatings are obtained, but certain improvements are desired.
Thus, although the latter patent teaches that the binder may be from 2% to 20%, in practice it has been found that a relatively high proportion of polymeric binder (at least 15%) is needed to help entrap the boron nitride in the coating. However, some of the higher amount of binder enters the coating and causes the as-sprayed coating to become too soft particularly after high temperature exposure. A lower binder content, even though producing good abradable coatings, results in relatively low deposit efficiency and higher hardness than desired.
If one of the constituents is formed of particles that are nearly the same size as the final powder, the composite is not homogeneous and, instead, comprises the larger particles as core particles with the finer second constituent bonded thereto by the binder. An example of such a clad powder is disclosed in U.S. Pat. No. 3,655,425 (Longo et al) wherein a constituent such as boron nitride is clad to nickel alloy core particles. The patent teaches that the core is only partially clad in order to expose core metal to the heat of the thermal spray process. Optionally, fine aluminum is added to the cladding for improvements that are speculated in the patent to be related to an exothermic reaction between the aluminum and the core metal.
Another powder for abradability comprises a core of a soft nonmetal such as Bentonite clad chemically with nickel alloy (without binder) as disclosed in U.S. Pat. No. 4,291,089 (Adamovic). U.S. Pat. No. 3,322,515 (Dittrich et al) teaches cladding metal core powders with aluminum subparticles using an polymeric binder.
U.S. Pat. No. 5,196,471 (Rangaswamy et al) discloses composite powders for thermal spraying of abradable coatings, in which the composite powders contain three components. One component is any of a number of metal or ceramic matrix materials, another component is a solid lubricant (such as a fluoride or boron nitride), and the third is a plastic. Although broad size ranges are disclosed for each component powder, specified as about 1 .mu.m to about 150 .mu.m, the only specific example (FIG. 1 of the patent) teaches fine particles of aluminum-silicon alloy and fine particles of CaF.sub.2 imbedded in the surface of a larger polymide core particle.
The basic and generally contrary goals of an abradable coating are to attain both abradability and resistance to gas and particle erosion. Resistance to the corrosive environments of a gas turbine engine also is required. Although existing coatings have been quite successful for the purpose, the exacting requirements are difficult to achieve in total, and searches for improved abradable coatings continue.